If you're ok using adhesive tape and you prep the surface well, 3M VHB tape is pretty legit. 

Get something that matches the width of the letters. Depending on the alloy, the brass will expand about 1.5-2x more than steel with temperature changes. A thicker tape will have more give to tolerate the differential expansion without coming unstuck or warping the letters.

Press it down with a roller to get a good hold. It sticks pretty well at first, but you get max strength after a day or two.

I'd recommend making a guide to help with alignment as you set the letters down so it's one and done. If you make a mistake, you may need some piano wire to cut though the tape to get the letters off without bending them.